1. Field of the Invention
The present invention relates to a technique for controlling flight characteristics or landing positions of liquid in a liquid-ejecting apparatus for ejecting the liquid contained in a liquid chamber from nozzles, and more specifically it relates to a technique for controlling a liquid-ejecting direction (liquid-landing position) from a liquid-ejection unit in a liquid-ejecting apparatus having a head where a plurality of the liquid-ejection units are juxtaposed to each other.
2. Description of the Related Art
An ink-jet printer has been known as an example of the liquid-ejecting apparatus having the head where a plurality of the liquid-ejection units are juxtaposed to each other. Also, a thermal system has been known as a system of the ink-jet printer for ejecting ink droplets using thermal energy.
As an example of the thermal-system printer-head chip structure, there is a structure in that ink in an ink chamber is heated by a heating element (heating resistor) so as to generate bubbles in the ink on the heating element, so that part of the ink is ejected as ink droplets by the energy produced during the bubbling. A nozzle is arranged above the ink chamber so that the ink droplets are ejected from a nozzle outlet when bubbles are generated in the ink contained in the ink chamber.
Furthermore, in view of the head structure, a serial system has been widely known in that the printer-head chips are moved in the width direction of photographic paper. Also, as is disclosed in Japanese Unexamined Patent Application Publication No. 2002-36522, a line system in that a large number of printer-head chips are arranged in the width direction of photographic paper so as to form a line head for the width of photographic paper is known.
FIG. 34 is a plan view of a conventional line head 10. In FIG. 34, four printer-head chips 1 ([N−1], [N], [N+1], and [N+2]) are shown; however, a further large number of the printer-head chips 1 are juxtaposed in practice.
Each printer head chip 1 is provided with a plurality of nozzles 1a having ejection openings for ejecting ink droplets. The nozzles 1a are juxtaposed in a specific direction, which agrees with the width direction of photographic paper. Furthermore, a plurality of the printer-head chips 1 are juxtaposed in a in a specific direction. In the printer-head chips 1 adjacent to each other, while the respective nozzles 1a are arranged so as to oppose each other, between the adjacent printer-head chips 1, the nozzles 1a are arranged so that the pitch thereof is sequential (see detailed portion A).
However, in the above-mentioned technique of Japanese Unexamined Patent Application Publication No. 2002-36522, when ink droplets are ejected from the printer-head chips 1, the ink droplets are ideally ejected normally to the ejection face of the printer-head chips 1; however, by various factors, the ejecting angle of the ink droplets may not be normal in practice.
For example, when a nozzle sheet having the nozzles 1a formed thereon is bonded on the upper surface of the ink chamber having the heating element, there arises a problem of a positional displacement between the ink chamber, the heating element, and the bonded position of the nozzle 1a. If the nozzle sheet is bonded so that the nozzle 1a is centered on the axes of the ink chamber and the heating element, ink droplets are ejected perpendicularly to the ejection face (the nozzle sheet surface). Whereas, if the nozzle 1a is not centered on the axes of the ink chamber and the heating element, ink droplets are not ejected perpendicularly to the ejection face.
Also, the positional displacement due to the difference in thermal expansion coefficient between the ink chamber, the heating element, and the nozzle sheet may be produced.
When ink droplets are ejected perpendicularly to the ejection face, it is assumed that the ink droplets be ideally landed at precise positions. If the ejecting angle of ink droplets is deflected by θ from the normal, when the distance H between the ejection face and photographic paper (landing surface of ink droplets) is constant (generally 1 to 2 mm in an ink-jet system), the positional displacement ΔL of the landing position of ink droplets is:ΔL=H×tan θ.
When such a displacement in an ejecting angle of ink droplets is produced herein, in the serial system, the landing pitch slippage of ink droplets appears between the nozzles 1a. In the line system, in addition to the landing pitch slippage, the deflection of the landing position appears between the printer-head chips 1.
FIG. 35 includes a sectional view and a plan view showing image-printing state in the line head 10 (a plurality of the printer-head chips 1 arranged in the arranging direction of the nozzles 1a) shown in FIG. 34. In FIG. 35, if the photographic paper P is assumed fixed, the line head 10 does not move in the width direction of the photographic paper P but it moves vertically in plan view so as to print images.
The sectional view of FIG. 35 shows the three printer-head chips 1 of Nth, (N+1)th, and (N+2)th printer-head chip 1, among the line head 10.
In the Nth printer-head chip 1, as shown by arrow of the sectional view, ink droplets are ejected slantingly in the left; also in the (N+1)th printer-head chip 1, in the right; and in the(N+2)th printer-head chip 1, as shown be arrow, ink droplets are ejected vertically without deflection.
Thus, in the Nth printer-head chip 1, ink droplets are landed at a deflected position in the left from a reference position; in the (N+1)th printer-head chip 1, in the right therefrom, so that ink droplets are landed at both positions receding from each other. As a result, between the Nth printer-head chip 1 and the (N+1)th printer-head chip 1, a region, on which no ink droplets are ejected, is formed. The line head 10 does not move in the width direction of the photographic paper P but moves only in arrow direction in plan view. Hence, between the Nth printer-head chip 1 and the (N+1)th printer-head chip 1, a white stripe B is produced, so that a problem has arisen that printed image quality is deteriorated.
In the same way as in the above-description, since in the (N+1)th printer-head chip 1, ink droplets are landed at a position deflected from the reference position in the right, between the (N+1)th printer-head chip 1 and the(N+2)th printer-head chip 1, a region where ink droplets are overlapped is formed. Thereby, there has been a problem that printed image quality is deteriorated by discontinuous images or a stripe C with a darker color than original one.
When the landing positional displacement of ink droplets is produced as described above, whether the stripe is conspicuous is affected by printed images. For example, a document has many blank portions, so that even if the stripe were produced, it is not so conspicuous. Whereas, when picture images are printed with full color on the almost entire region of photographic paper, even when a slight stripe is produced, it becomes conspicuous.
In order to prevent the stripe described in FIG. 35 from being produced, Japanese Unexamined Patent Application Publication No. 2002-240287, to the same assignee, proposes a technique.
In Japanese Unexamined Patent Application Publication No. 2002-240287, a plurality of the heating elements (heaters), which can be independently driven, are provided within the ink chamber, so that the ejection direction of ink droplets can be changed by independently driving each heating element. It has been considered that the generation of the stripe (white stripe B or stripe C) is solved by the technique of Japanese Unexamined Patent Application Publication No. 2002-240287.
In Japanese Unexamined Patent Application Publication No. 2002-240287, the ejection direction of ink droplets is deflected by independently controlling a plurality of heating elements; however, with the examination thereafter, when this technique is adopted, ink droplets may be ejected unstably, so that a problem has been proved in that high-quality images cannot be stably obtained.
According to the investigation by the inventors, in general, the election amount of ink droplets from the liquid ejection part does not simply increase with increasing electric power applied to the heating element, so that the ejection is not performed until a predetermined amount of electric power is applied thereto. In other words, if a predetermined amount of electric power or more is not applied, a sufficient amount of ink droplets cannot be ejected.
Hence, when a plurality of heating elements are independently driven, if ink droplets are ejected by driving only some parts of the heating element, a sufficient calorific value required for ejecting ink droplets must be generated only by this parts of the heating element. Thus, when a plurality of heating elements are independently driven, and ink droplets are ejected by driving only some parts of the heating element, it is necessary that electric power applied to the parts of the heating element be increased. Such situation is unfavorable for the miniaturization of the heating element with the recent progress to higher resolution.
That is, in order to stably eject ink droplets, a yield of energy per unit area of each heating element must be increased than before. As a result, the miniaturized heating element may be damaged more badly, thereby reducing the life of the heating element as well as of the head.
In conclusion, in the head having the heating element miniaturized with the progress to higher resolution, the stripe cannot be prevented from being generated with the above-described various techniques.